High efficiency linearization power amplifier for wireless communication

ABSTRACT

A predistortion system for linearizing the output of a power amplifier includes a first signal representative of an RF modulated signal and a feedback signal representative of nonlinear characteristics of a power amplifier. The system also includes a predistortion controller, comprising at least one lookup table, adapted to receive the first signal and the feedback signal and to generate a correction factor for correcting the nonlinear characteristics of the power amplifier and combining logic which combines the RF modulated signal with a signal corresponding to the correction factor and supplies it to the power amplifier to linearize the output of the power amplifier.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/799,239, filed on Apr. 30, 2007, which is a continuation-in-part ofU.S. patent application Ser. No. 11/262,079, filed on Oct. 27, 2005,which is a continuation of U.S. patent application Ser. No. 10/137,556,filed May 1, 2002, now U.S. Pat. No. 6,985,704. The disclosures of eachare hereby incorporated by reference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

A fundamental component of wireless communications systems is the poweramplifier (PA). Such wireless communications systems include a varietyof broadband and multimedia services. In such systems, the poweramplifier is supported by a wireless RF transmitter. The powerefficiency of the PA is an important aspect of the operationalefficiency of the system as a whole. For a long time, the linearity andefficiency of power amplifiers have been two of the most difficultissues faced by wireless system designers and users. These problems areclosely related to both the power spectrum efficiency and the powerefficiency of power amplifiers.

Prior art predistortion technologies used to linearize the PA inwireless communication systems mainly use analog predistortionapproaches. Some have also used a digital adjustment componentimplemented by means of an analog feed-forward approach and a digitalpredistortion approach employing a base-band digital signal processing(DSP) technique.

The traditional analog predistortion technologies are based on theprinciple of error subtraction and a power match to linearize the PA.These approaches must use an auxiliary power amplifier to match exactlythe non-linearity of the main PA and, at the same time, overcome thesignal time-delay that is a variable with frequency and environmentaltemperature. As a result, it is difficult for this approach to meet therequirement of the advanced wireless communication systems.

Prior art DSP-based predistortion methods generally require that thepredistortion sub-system and circuit design rely on the I and Q signalof the base band in the base station, and therefore have to embed thepredistortion processing into the base band circuits of the basestation. This requires that the existing base band circuit in the basestation must be modified, which is an inconvenient high-cost solutionfor wireless carriers and equipment makers. Also, the typical DSP-basedpredistortion technology is not suitable to linearization of the PA whenmulti-modulation schemes and multi-carrier signals go through a PA.

SUMMARY OF THE INVENTION

The present invention includes methods and apparatus for improving powerefficiency and linearity of a PA by pre-distorting the input signal tothe PA. By applying the methods of the present invention to pre-distortthe signal, the conversion characteristic of the normally nonlinear PAbecomes substantially linear. As a result, the PA uses power moreefficiently and provides advanced performance characteristics suitablefor wireless signal transmission. The present invention can be appliedto a broad array of wireless communication systems, regardless ofmodulation type, to yield high quality signal transmission and expandednetwork capacity. It will be appreciated by those skilled in the artthat the power amplifiers described herein can also be regarded as highpower amplifiers.

In an embodiment of the invention, a predistortion controller comprisesanalog peripheral circuits and a predistortion core processor for RFsignal predistortion processing in preparation for PA signaltransmission from a wireless communication device.

The analog peripheral circuits in the predistortion controller comprisethe analog down-converter circuits prior to the PA input and the outputof the PA, respectively, to pick up and compare an idealized signal fromthe modulator in the wireless system to the distorted feedback signalfrom PA's output. The predistortion core processor operates togetherwith a look-up table, where the look-up table stores predistortioninformation that is obtained by mathematically calculating AM-AM andAM-PM distortion components. The output of the predistortion processorprovides a pre-correction signal for the input of the PA. Theprecorrection signal is combined with the signal from the RF modulatorby a multiplication operation to generate a pre-distorted input signalthat is provided as the input to the PA.

The basic architecture of the predistortion sub-system can be betterunderstood from the appended Figures, as set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the PA module with a predistortioncontroller in accordance with an embodiment of the invention.

FIG. 2 presents a simplified architecture of an embodiment of thepredistortion controller of FIG. 1.

FIG. 3 illustrates in greater detail the embodiment of FIG. 2.

FIG. 4 shows the operation and arrangement of the lookup table in thepredistortion controller shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to wireless communicationssystems, and more particularly relates to systems, apparatus and methodsfor correcting the nonlinearity of a Power Amplifier (PA) such as mightbe used in a wireless RF transmitter.

The present invention provides a novel predistortion controller tolinearize the power amplifier (PA) used in wireless base stationsoperating in wireless networks using a wide variety of signal typesincluding CDMA, TDMA, GSM, GPRS, 3G systems (UMTS, W-CDMA, CDMA2000,TDS-CDMA, 3GPP and others), WLAN, WiFi, WiMax and the like, as well asother proposed and upcoming wireless systems (4G/5G) that transmit acomplex modulated signal by using a high power amplifier. Because thehigh power amplifier in wireless RF transmitters typically distorts theRF output signal, the adaptive predistortion controller of the presentinvention is useful to correct that non-linearity. Embodiments of thepresent invention are suitable for use with ail wireless base stations,access points and other wireless communication systems such as microwaveand satellite communications. The predistortion controller of thepresent invention offers the additional advantage that it can be addedto existing wireless RF transmitters without otherwise changing ormodifying the base station structure while at the same time yieldingsignificant performance improvements. Included in these improvements isa substantial increase in the efficiency of the base station, whichyields a significant reduction in power usage because, in currentsystems, significant power is wasted due to the nonlinearcharacteristics of typical PA's.

From the following description, it will be appreciated that thehigh-efficiency linearization predistortion controller disclosed hereinoffers the following benefits to the wireless industry:

1. It significantly enhances the transmitted signal quality andincreases wireless network capacity by improving the PA's transmissioncharacteristics and decreasing the adjacent channel interference.

2. It can be implemented as an add-on module to the PA, which permitsthe aforementioned improvements to the performance of the PA withoutchanging or modifying the existing RF and base band circuits in the basestation. This differs from the traditional feedback and feed-forwardpredistortion approach, and therefore is a comparatively low cost, easyto install solution for the wireless network infrastructure whenever theoperators desire to upgrade performance and reduce energy costs.

3. The predistortion control operation is fast and dynamic, whichpermits it to track and correct for nonlinearities of the PA in a wider,and makes it particularly suitable for use in base stations, repeatersand handsets.

4. It can be integrated into the design of the PA for convenientinstallation and replacement of aging PA's already installed in awireless system.

Referring first to FIG. 1, the basic arrangement of the presentinvention can be appreciated. A predistortion controller 5 received asan input a modulated RF signal 6 and also receives a feedback signal 7which correlates to the output signal of a power amplifier 12. Thepredistortion controller modifies the RF signal 6 in accordance with thenonlinear and distortion characteristics of the output signal of thepower amplifier 12 as characterized by the feedback signal 7, such thatthe output of the predistortion controller 5 is supplied to the poweramplifier 12 to yield a substantially linearized and improved out putsignal of the power amplifier 12.

Referring next to FIGS. 2 and 3, an embodiment of the present inventionmay be appreciated in greater detail. More particularly, the illustratedembodiment includes an Analog Multiplier I1 which receives a modulatedRF signal V_(RF) from the RF modulator portion 10 [FIG. 3] of the basestation, and also receives a predistortion correction signal V_(p) froma predistortion processor 200, shown generally in FIG. 2 and in greaterdetail in FIG. 3, and discussed in greater detail below. The output ofthe analog multiplier 11 is provided as the input V_(in) to the poweramplifier (PA) 12, which in turn transmits an output signal V_(o) to anAntenna 13. The RF modulator 10 is typically although not necessarily aquadrature modulator or an orthogonal modulator. It will be appreciatedthat multiplier 11 can be implemented as multiple multipliers, eachassociated with one or more quadrature signals.

An input down-converter circuit 20 receives an idealized referencesignal V_(RF) from modulator in base station, and is biased by a localoscillator 40, such that it provides an output V_(ct) to ananalog-to-digital converter 21. The ADC 21 converts the signal V_(ct) todigital form, whereupon it is provided as one input to a DigitalPredistortion Processor 200.

A feedback down-converter circuit 26, also biased by a local oscillator40, receives a raw feedback signal V_(o)(t) from the output of the PA,and provides a feedback signal V_(f) to an feedback ADC 25. The digitaloutput of the ADC 25 then provides a second input, i.e., feedbacksignal, to the Digital Predistortion Processor 200. The DigitalPredistortion Processor 200, discussed in greater detail below, providesa digital output signal V_(r) to a DAC 30, which converts the digitsignal to an analog form, where it is combined with the modulated RFsignal in the multiplier 11.

As shown in FIG. 3, address data formers 32I-32Q receive inputs from theADC 21I/Q, and are designed to generate the required signal format for alookup table 33I/Q. The data formers 32I/Q address memory units withinthe lookup tables 33I/Q, where the lookup table provides separate I andQ outputs to an adder 31. It will be appreciated that the lookup table33 can be implemented as one or more lookup tables. The address providedby the address formers 32I-32Q can be considered a lookup-table key oraddress.

The predistortion controller lookup tables 33I-33Q are designed memoryunits to store the predistortion signal for high power amplifierlinearization. The predistortion signals in the tables are based on theerror generated by a comparison of the ideal signal V_(d) and thefeedback signal V_(f) and the presented adaptive algorithm. The datastored in the tables 331-Q can be updated by adaptive iteration asdescribed hereinafter, and forms digitally indexed data reflecting thenonlinear characteristics of the power amplifier.

By comparison of AM-AM and AM-PM information between the idealizedsignal V_(RF)(t) and the feedback signal Vo(t), the DigitalPredistortion Processor calculates the error in the amplitude and phasecomponents of the output signal V_(o)(t) caused by the non-lineartransmission characteristics of the high power amplifier 12.

Based on the error information obtained by the foregoing comparison, thepredistortion processor, based on the lookup table algorithm disclosedin U.S. Pat. No. 6,985,704, the disclosure of which is herebyincorporated by reference, calculates and generates adaptively acompensation signal that is of inverse characteristics with thetransform function of the PA 12 to pre-distort the AM-AM and AM-PMdistortion caused by the PA 12.

The outputs V_(p) of the predistortion lookup table 331-33Q are fed tomultiplier 11, after an adder 31 and a digital-to-analog converter 30,to modify the modulated RF signal from modulator 10. The output of themultiplier is the required predistortion signal V_(in)(k) that is of aninverse non-linearity with that of the power amplifier 12 to yield apre-compensation to the input of high power amplifier.

It will be appreciated by those skilled in the art that there can be asignal difference between two signals, ideal signal V_(RF) and feedbacksignal, V_(o)(t), when they arrive at predistortion controller 5 [FIG.1], or, in FIG. 2, at processor 200. The time difference results fromthe time-delay differences between the two signals, which is caused bythe different paths each travels in arriving at the processor 200. Thissignal time-delay can vary randomly based on the parameters of thecircuits and parts, as well as other environmental factors. The resultis that it is difficult to estimate, calculate and adjust for suchsignal differences in the field application environment. To overcomethis issue, the present invention adaptively adjusts for this time-delaythrough the use of an algorithm taught by the previously cited U.S. Pat.No. 6,985,704.

The use of the look-up tables 33 permits a memory function to beintroduced into at least some embodiments of the present invention. Thelookup table of the predistortion controller is based on a storedcompensation principle that maps a set of input data into a digitaloutput, and updated adaptively. Based on the stored function, eachoutput signal of lookup table is actually related to both the currentand the previously transmitted signal, and therefore has a memoryfunction which compensate not only for the non-linearity of the PA, butalso avoids the need for a special time-delay compensation circuit suchas typically used in the prior art. See particularly U.S. ProvisionalPatent Application No. 60/898,312, filed Jan. 29, 10 2007, entitledPower Amplifier Time-delay Invariant Predistortion Methods andApparatus.

The architecture of an embodiment of a lookup table which can implementthe compensation principle discussed above is shown in FIG. 4. The datafrom ADC 21 is supplied to address former 32, which in turn forms anaddress and applies it to the look-up tables 33. At the same time, thefeedback signal V_(feedback) from ADC 25 and the ideal signal V_(ideal)from ADC 21 are compared in the error generator 23 and the resultingerror signal is multiplied by a numerical value I1 in multiplier 35. Thevalue of I1 is typically between 0 and 1, and represents a convergencefactor that can be better understood from the teachings in U.S. Pat. No.6,985,704. The scaled error factor is then added with a feedback factorin adder 34 and supplied back to the lookup table 33. The result is anoutput of the lookup table which provides compensation for the timedelay as well as nonlinearities of the PA 12.

Due to introducing the memorized lookup table processing, there isunnecessary to build another special time-delay circuit for the signaldelay processing. Therefore, the memorized lookup table in this patentshows two functions, the nonlinear predistortion of high power amplifierand adaptive signal time-delay adjusting.

The lookup table of predistortion controller is based on a storedcompensation principle that maps a set of input data into a digitaloutput, and are updated adaptively. Based on the stored function, eachoutput signal of lookup table is actually related to both the currentand previous transmitted signal, and therefore has a memory functionwhen compensate the non-linearity of PA [1]. The architecture of lookuptable based on the compensation principle is shown in FIG. 4.

It will be appreciated further that the performance of the predistortioncontroller is, in some respects, related to the number of bits in theanalog-to-digital converters in the original and feedback channels,whereby the bigger the number of bits in the ADC, the better theperformance or the predistortion controller is. Similarly, the memoryfunction of the predistortion controller is also related to the numberof bits in the address-shifting register, such that, within reasonablesizes, the performance of the predistortion controller improves as thenumber of bits in the register increases. It will further be appreciatedthat the PA may be operated in any region, such that, for example, itsbias or static operation point can be set either in the saturationregion or the cut-off region, with appropriate adjustments for operationin each region. It will further be appreciated that the output signal ofpredistortion controller is a stochastic control signal rather than amodulated high-frequency signal, and the signal is of the inversecharacteristic with the AM-AM and AM-PM distortion component of poweramplifier. Those skilled in the art will also appreciate that the outputsignal of the predistortion controller is not a high-frequency signal,and its frequency typically match the bandwidth of the signal to betransmitted by a power amplifier in a wireless transmitter.

The combination of both memory and store function performed by thelookup table extends the ability of the present system to compensate forthe PA's non-linear characteristics to time-independent aspects. Thetime-independent feature of the lookup table's adaptive processing is akey benefit of the addressing arrangement of the lookup table. In anembodiment, the addressing of the lookup table is implemented by a setof N-bit vector data that contains the current input signal and previousN input signal as well. Therefore, the address of the lookup table is acombination of series of input sequences with the length of N. Thelonger the address of the lookup table, the wider the time-delayinformation range that the system can accommodate (i.e. the longerduration of time-delay effects that the system can tolerate).

Having described the invention in detail, including several embodimentsand alternatives, those skilled in the art will appreciate that numerousother alternatives and equivalents exist which are within the scope ofthe present invention. Therefore the invention is intended not to belimited by the above description, but rather only by the appendedclaims.

1. A predistortion system for linearizing the output of a poweramplifier comprising: a first signal representative of an RF modulatedsignal; a feedback signal representative of nonlinear characteristics ofa power amplifier; a predistortion controller, comprising at least onelookup table, adapted to receive the first signal and the feedbacksignal and to generate a correction factor for correcting the nonlinearcharacteristics of the power amplifier; and combining logic whichcombines the RF modulated signal with a signal corresponding to thecorrection factor and supplies it to the power amplifier to linearizethe output of the power amplifier.